The present invention generally relates to a diagnostic device for use with process control systems. More specifically, the present invention relates to hand-held diagnostic devices for interfacing with a packet based network (such as Fieldbus) in a process control system and for interacting with the communications of the field devices linked to the network in order to associate communication packets with specific field devices for the purpose of diagnosing problems on the network.
In industrial settings, control systems monitor and control inventories, industrial and chemical processes, and the like. Typically, the control systems perform these functions using field devices distributed at key locations in the industrial process and coupled to the control circuitry in the control room by a process control loop. The term “field device” refers to any device that performs a function in a distributed control system, including all devices currently known in the control art.
Generally, each field device includes a transducer or actuator. A transducer (including actuators) is understood to mean either a device that generates an output signal based on a physical input or that generates a physical output based on an input signal. Typically, a transducer transforms an input into an output having a different form. Often, one system provides power to actuate a transducer, which in turn supplies power usually in another form to a second system. For example, a loudspeaker is a transducer that transforms electrical signals into sound energy. Types of transducers typically include various analytical equipment and sensors, pressure sensors, thermistors, thermocouples, strain gauges, flow transmitters, positioners, actuators, solenoids, indicator lights, and the like.
A communication network links the controllers with the sensors and actuators located in the field. One such communications network is a packet based network such as Fieldbus. Fieldbus is a generic term used to describe a digital, bidirectional, multidrop, serial communication network for connecting isolated field devices, such as controllers, actuators and sensors, in industrial applications. The Instrument Society of America (ISA) adopted a standard for Fieldbus communications, which is identified as ISA SP50.02. The ISA standard Fieldbus system utilizes a two wire bus to provide simultaneous digital communication and DC power to remotely located field devices.
In general, the Fieldbus communication protocol is a known open communication protocol that is employed in many process control networks. Generally, Fieldbus provides synchronous (i.e., scheduled) communications and asynchronous (e.g. token ring) communications on a network bus. The scheduled/synchronous communications are used for signals relating to actual process control activities, while the asynchronous communications are used to convey secondary information (such as communications to and from a user or for measurements not directly related to process control).
Field devices derive power from the two-wire bus, and transmit data to and receive messages from a central controller, which is separated physically from the field devices. When problems arise in communicating with a particular field device or if data received from the field device is anomalous, it is sometimes necessary to diagnose the problem manually.
Some attempts have been made to incorporate diagnostic circuit elements within individual field devices to allow the field devices to diagnose themselves. However, such additional circuitry increases the cost of the individual field device. Additionally, when transmission errors arise, such internal circuitry may be insufficient to properly identify the problem, sending messages indicating the field device is okay when in fact there is a problem. Moreover, such extra circuitry places a power drain on the Fieldbus power supply that most of the time is unnecessary, resulting in wasteful power dissipation.
It is also possible to connect a device, such as a Fieldbus Monitor FBT-3 produced by RELCOM, Inc. of Forest Grove, Oreg., to examine a “live” Fieldbus network without interfering with its operation. Such a device is intended to verify and to troubleshoot network operation, but, by itself, it cannot communicate on the Fieldbus segment.
As previously mentioned, Fieldbus is a type of serial communication network for connecting isolated field devices, such as controllers, actuators and sensors or transmitters, in industrial applications. Typically, the network involves homerun cabling connecting a control center with various field devices, either directly or via junction boxes, which connect one or more serial devices to the network. Generally, a fieldbus capable field device includes a fieldbus interface chip and a sensor, transducer, actuator or other similar circuit element for functioning on the network.
Since the precise requirements of each field device vary, it is not always possible to provide a direct interface from the Fieldbus Interface Chip to the measurement circuit of the field device. Moreover, on a “live” Fieldbus network, multiple data-link-protocol-data-units (DLPDUs) may be traversing the communication path in quick succession. This requires that the diagnostic circuit be capable of correctly associating a DLPDU with its source.
With current diagnostic devices and methods, troubleshooting communication problems and configuration errors associated with the field devices on the protocol bus is difficult. As a result, users at the system level cannot easily identify the source of problems and are typically only informed by the system that the overall process is not operating properly. When such an alert arises, it is difficult to identify and fix the errant field device.